For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel tendon, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there are provided anchors for anchoring the ends of the tendons suspended therebetween. In the course of installing the tendon tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of the tendon for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the tendon, wedges, threaded nuts, or the like, are used to capture the tendon and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such as de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the tendon or anchor suffer corrosion, then they may become weakened due to this corrosion. The deterioration of the anchor or tendon can cause the tendons to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
Several U.S. patents have considered the problem of anchor and tendon corrosion. For example, U.S. Pat. Nos. 4,896,470 and 5,072,558 disclose tendon tensioning anchor systems in which the metal anchor for the system is encapsulated in plastic and has a tubular portion extending outwardly towards the surface of the post-tensioned concrete body. A sealing cap is fitted to the end of the tubular portion of the plastic encapsulation to provide a fluid tight seal for protecting the post-tensioned tendon, anchor and tensioning wedges from exposure to the elements. Other prior art systems also exist in which the end of the post-tensioned tendon is severed at a point inwardly from the outer surface of the post-tensioned concrete body and means are used to protect the tendon end, anchor and tensioning wedges from exposure to the elements.
When using prior art systems for corrosion protection of the tensioning tendon and related apparatus, it is important that the tendon be terminated at a point inboard from the outside surface of the post-tensioned concrete body. This requires that the end of the tendon be cut just outboard of the tensioning wedges and within the pocket or cavity formed by the pocket former. The most common method used in the prior art for the cutting of such tensioned tendon at this point is an acetylene torch or cutting torch. Normally, the end of the cutting torch is placed in close proximity to the face of the anchor and within the pocket. Heat is applied directly to the tendon so as to sever the tension from that portion received within the anchor. Unfortunately, the cutting of the tension with a torch at the point near the tensioning wedges can cause the tendon and wedges to become heated and can result in the loss of temper of the metal or loosening of the post-tension wedges. Alternatively, the torch is not brought into close enough proximity to the anchor such that an improper cutting of the tension occurs. In other words, the tension may be cut so that an end of the tendon extends outwardly of the pocket and outwardly of the concrete body. No techniques have been used in the past for placing such a torch in close proximity to the anchor body within the pocket without causing the torch to adversely affect the post-tensioning wedges or the integrity of the anchor.
Another technique used for the cutting of the tendon is a conventional electric saw. However, this requires that a portion of the slab or other concrete structure surrounding the anchor also be cut in order to reach the portion of the tendon which is within the pocket formed in the concrete adjacent to the anchor.
Importantly, U.S. Pat. No. 5,436,425, issued on Jul. 25, 1995 to the present inventor described a system whereby the tendon could be properly cut by using a plasma cutting torch. This method and apparatus utilized a positioning element for interconnecting the head of a plasma cutting torch with the tendon to be severed. A positioning element conforms in shape to the pocket in the concrete body adjacent to the anchor. As such, the positioning element can be placed over the tendon and pushed into the pocket prior to activating the plasma cutting torch for the purposes of severing the tendon. The plasma cutting torch utilizes an air compressor, an electrical power source and electrodes to generate a stream of air, nitrogen or other gaseous plasma at very high temperatures of about 3000.degree. C.-5000.degree. C. or more. The stream of superheated gaseous plasma cuts through the metal more quickly than in an acetylene torch. The positioning element included a combination clamp and shield member which is releasably engagable with the tendon. The clamp included an attachment means or bracket which is engagable with the cutting head of the plasma torch. The attachment bracket is appropriately sized so as to be friction fitted around the cutting head or could be permanently attached to the cutting head. The positioning element is attached to a bracket and is releasably engagable with the tendon to be cut for positioning the cutting tip a predetermined distance from the longitudinal axis of the tendon. The positioning element includes a pair of depending jaws pivotally attached to a mounting bracket and biased towards a closed position by a spring. Each of the jaws has a semi-circular relieved portion at its inner edge which, when together, provides a circular opening through which the tendon may be received. Once the tendon is received in the center opening, the jaws close upon it.
Unfortunately, in this prior art patented device, it was found that typical construction environments do not utilize plasma cutting torches. These plasma cutting torches are very expensive and are typically not operated in a proper manner by the construction workers. As such, strong resistance to the use of such a plasma cutting torch has occurred. Conventionally, construction workers continue to utilize acetylene torches for the severing of the tendon.
Another problem associated with the use of torches for the cutting of tendons associated with plastic-encapsulated anchors used in a post-tension anchor system is that the cutting torch can impart heat to the tendon to such a sufficient degree as to cause a melting of the encapsulation. Since it is important to maintain the post-tension anchor system in an encapsulated condition, all of the plastic components of such an encapsulated system must be properly maintained. If, for example, the cap-receiving portion of the encapsulated anchor should become deformed or melted by heat, it would be impossible to attach the necessary cap or sealing devices so as to assure that the post-tension system is properly sealed. As such, it is important to be able to avoid the melting of the plastic encapsulations.
After use, it was discovered that the product associated with U.S. Pat. No. 5,436,425 effectively shielded the anchor from the heat imparted by the cutting torch. As such, it prevented the deterioration in the structural integrity of the tendon or the securing wedges. Unfortunately, it was found that under such heated conditions, the grease associated with the encapsulated tendon could become ignited and create a flame which would migrate along the tendon into the area of the plastic encapsulation. As such, a need developed so as to prevent this migration of such heat and flames from entering the area of the plastic encapsulation.
In certain countries, the use of cutting torches for the severing of the tendon is prohibited because of the fire risk associated with the use of such cutting torches. Since construction sites often contain many flammable materials or combustible materials, the use of such acetylene torches can create a fire hazard. As such, a need developed so as to prevent any sparks from exiting the cutting area. When the cutting torch is inserted into the pocket associated with the anchor of the post-tension anchor system, sparks will fly from the pocket as the tendon is severed. It is important to be able to contain such sparks and to prevent such sparks from possibly creating a fire hazard at the construction site.
It is, therefore, the primary object of the present invention to provide a method and apparatus for severing the free end of a post-tensioned tendon at a point near the tensioning wedges and within the depth of the pocket formed for the anchor member.
Another object is to provide such a method and apparatus in which the tendon is cut without substantially heating the tendon and tensioning wedges.
A still further object is to provide such a method and apparatus in which the tendon can be cut at the desired location without damaging the post-tensioned concrete body.
It is an object of the present invention to provide a method and apparatus in which the tendon can be cut by using a conventional acetylene torch.
It is another object of the present invention to provide an apparatus which prevents the migration of heat and flames to the encapsulation associated with an anchor body.
It is another object of the present invention to provide a method and apparatus which prevents sparks from flying from the pocket formed in the cavity associated with the construction anchor.
It is still object of the present invention to provide a cutting torch which is easy to use, relatively inexpensive, and simple to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.